Multipathing (MPIO) software allows the hosts to maintain multiple paths to a LUN. Paths to the NetApp node where a LUN resides are considered "Active/Optimized" while paths through the node's HA partner are considered "Active/Un-Optimized". The exact terminology may vary depending on the host operating system and multipath drivers. Certain scenarios can lead to one or more paths being inaccessible or otherwise unavailable for I/O. Depending on which paths are unavailable, the host may experience performance degradation, especially if there are no other "Active/Optimized" paths remaining.
Path states are determined by the node that currently owns a LUN using ALUA. When a takeover occurs, the paths through the node that assumes ownership of it's partner's LUNs will become "Active/Optimized".
Example of a path failure in RedHat 'multipath -ll' output:
# multipath -ll
[size=10 GB][features="1 queue_if_no_path"][hwhandler="0"]
_ round-robin 0 [prio=8][active]
_ 0:0:0:0 sda 8:0 [failed][faulty]
_ 1:0:0:0 sdi 8:128 [active][ready]
_ round-robin 0 [prio=2][enabled]
_ 0:0:1:0 sde 8:64 [active][ready]
_ 1:0:1:0 sdm 8:192 [active][ready]
RedHat '/var/log/messages' Examples:
Jun 22 15:55:54 rhel1 multipathd: Lun1: sdcr - tur checker reports path is down
Jun 22 15:55:54 rhel1 multipathd: checker failed path 69:240 in map Lun1
Jun 22 15:55:54 rhel1 multipathd: Lun1: remaining active paths: 3
Jun 22 15:55:54 rhel1 kernel: device-mapper: multipath: Failing path 69:240.
VMware 'vmkernel' Examples:
2017-11-30T07:17:41.351Z cpu15:33143)WARNING: NMP: nmp_DeviceRequestFastDeviceProbe: NMP device "sym.029025256531353837" state in doubt; requested fast path state update...
2017-11-30T07:17:41.351Z cpu15:33143)WARNING: ScsiPath: 4507: Path vmhba2:C0:T0:L55 is being removed
2017-11-30T07:17:41.351Z cpu15:33143)WARNING: ScsiPath: 6117: Remove path: vmhba2:C0:T0:L55
Windows 'System Event Log' Examples:
Information 10/31/2017 4:39:33 PM mpio 46 None Path 1000001 was removed from \Device\MPIODisk2 due to a PnP event. The dump data contains the current number of paths.
Information 10/31/2017 4:39:33 PM ontapdsm 61070 None Path ID 01000001 was removed.
Warning 10/31/2017 4:39:33 PM ontapdsm 61054 None Path ID 01000001 has failed and 02000101 will be used instead.
Error 10/31/2017 4:39:33 PM ontapdsm 61142 None Path ID 01000001 has failed.
Note: The exact log entries may vary depending on the operating system version and how multipathing is configured.